Me pehea te whakahaere a MySQL i nga patai uara kore-kore? Ehara a MySQL i te tauākī kōwhiri kore

pātengi raraunga MySQLMe pehea te whakahaere i nga patai uara kore-kore?MySQL ehara i te korero kowhiri kore

Te whakahaere kore i MySQL

Kua mohio tatou kei te whakamahi a MySQL i te whakahau SQL SELECT me te WHERE rara hei panui i nga raraunga i roto i te ripanga raraunga, engari i te mea ko te NULL te waahi patai e whakaratohia ana, kaore pea te whakahau e mahi tika.

Hei whakahaere i tenei ahuatanga, ka whakaratohia e MySQL nga kaiwhakahaere nui e toru:

• HE NULL:Ka hoki mai tenei kaiwhakahaere i te pono ina he NULL te uara o te pou.
• KARERE NULL:Ka hoki mai te kaiwhakahaere i te pono ka kore te uara o te pou i te NULL.
• <=>:  Ko te kaiwhakahaere whakatairite (kaore i rite ki te = operator) ka hoki pono ina he NULL nga uara e rua.

He mea motuhake nga mahi whakataurite herenga i runga i te NULL.Kaore e taea e koe te whakamahi = NULL ranei! =Ka kitea e NULL nga uara NULL i te pou.

I roto i MySQL, ko te whakatairite o te uara NULL me tetahi atu uara (ahakoa NULL) ka whakahoki teka i nga wa katoa, ara ko NULL = NULL ka hoki teka.

Ka whakahaerehia te NULL i MySQL ma te whakamahi i te IS NULL me te IS NOT NULL.

Whakamahia te uara NULL i te whakahau whakahau

I roto i te tauira e whai ake nei, ko te ripanga chenweiliang_test_tbl i roto i te chenweiliang pātengi raraunga kua whakaritea kia rua nga pou, chenweiliang_author me chenweiliang_count, me nga uara NULL ka whakauruhia ki te chenweiliang_count.

Akoranga

Whakamātauria ngā tauira e whai ake nei:

root @ host ＃mysql -u root -p password; 输入密码：*******
 mysql > 使用chenweiliang ;
数据库改变了mysql > create table chenweiliang_test_tbl 
 - > （
 - > chenweiliang_author varchar （40 ）NOT NULL ， - > chenweiliang_count INT 
 - > ）;
查询OK ，0 行受影响（0.05 秒）mysql >
 
 
INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （' chenweiliang ' ，20 ）;
mysql > INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （' 陈沩亮博客' ，NULL ）;
mysql > INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （ ' Google ' ，NULL ）;
mysql > INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （' FK ' ，20 ）;
 
mysql > SELECT * from chenweiliang_test_tbl ; + --------------- + -------------- + | chenweiliang_author | chenweiliang_count | + --------------- + -------------- + | chenweiliang | 20 | | 陈沩亮博客| NULL | | Google | NULL | | FK | 20 | + --------------- + -------------- +
 4 行中集合（0.01 秒） 

I roto i te tauira e whai ake nei ka kite koe = me ! Ko te = karekau e mahi:

mysql > SELECT * FROM chenweiliang_test_tbl WHERE chenweiliang_count = NULL ;
空集（0.00 秒）mysql > SELECT * FROM chenweiliang_test_tbl WHERE chenweiliang_count ！= NULL ;
空集（0.01 秒）

Hei kimi mehemea he NULL te chenweiliang_test_tbl tīwae i te ripanga raraunga, me whakamahi koeIS NULLAKORE NULL, te tauira e whai ake nei:

mysql > SELECT * FROM chenweiliang_test_tbl WHERE chenweiliang_count IS NULL ; + --------------- + -------------- + | chenweiliang_author | chenweiliang_count | + --------------- + -------------- + | 陈沩亮博客| NULL | | Google | NULL | + --------------- + -------------- +
 2 行中的组（0.01 秒）的MySQL > SELECT * 从chenweiliang_test_tbl WHERE chenweiliang_count IS NOT 空值 
 
 ; + --------------- + -------------- + | chenweiliang_author | chenweiliang_count | + --------------- + -------------- + | chenweiliang | 20 | | FK | 20 | + --------------- + -------------- +
 2 行中的组（0.01 秒） 

Te whakahaere i nga uara NULL me te tuhinga PHP

I roto i te tuhinga PHP, ka taea e koe te whakamahi i te korero if...else ki te tukatuka mehemea kei te putua te taurangi me te whakaputa i tetahi tauākī herenga.

I roto i te tauira e whai ake nei ka tautuhia e PHP te taurangi $chenweiliang_count ka whakamahi i taua taurangi hei whakataurite ki te mara chenweiliang_count i te ripanga raraunga:

<?
php $ dbhost = ' localhost：3306 ' ; // mysql服务器主机地址

$ dbuser = ' root ' ; // mysql用户名
$ dbpass = ' 123456 ' ; // mysql用户名密码
$ conn = mysqli_connect （$ dbhost ，$ dbuser ，$ dbpass ）;
如果（！$ conn ）{ die （' 连接失败：' 。mysqli_error （$ conn ））;
} // 设置编码，防止中文乱码

mysqli_query （$ conn ，“ set names utf8 ” ）;
 
if （isset （$ chenweiliang_count ））{ $ sql = “ SELECT chenweiliang_author，chenweiliang_count FROM chenweiliang_test_tbl WHER chenweiliang_count = $ chenweiliang_count ” ;
} else { $ sql = “ SELECT chenweiliang_author，chenweiliang_count FROM chenweiliang_test_tbl WHER chenweiliang_count IS NULL ” ;
} mysqli_select_db （$ conn ，'


 chenweiliang ' ）;
$ retval = mysqli_query （$ conn ，$ sql ）;
if （！$ retval ）{ die （' 无法读取数据：' 。mysqli_error （$ conn ））;
} echo ' <h2>陈沩亮博客IS NULL测试<h2> ' ;
echo ' <table border =“1”> <tr> <td>作者</ td> <td>登陆次数</ td> </ tr> ' ;

 $ retval ，MYSQL_ASSOC ））{ echo “ <tr> ” 。
 “ <td> {$ row ['chenweiliang_author']} </ td> ” 。
 “ <td> {$ row ['chenweiliang_count']} </ td> ” 。
 “ </ tr> ” ;
} echo ' </ table> ' ;
mysqli_close （$ conn ）;
?>